Hearing aid users have been reported to have poorer ability to localize sound sources when wearing their hearing aids than without their hearing aids. This represents a serious problem for the hearing impaired population.
Furthermore, hearing aids typically reproduce sound in such a way that the user perceives sound sources to be localized inside the head. The sound is said to be internalized rather than being externalized. A common complaint of hearing aid users trying to understand speech in noise is that it is very hard to follow anything that is being said even though the signal to noise ratio (SNR) should be sufficient to provide the required speech intelligibility. A significant contributor to this fact is that the hearing aid reproduces an internalized sound field. This adds to the cognitive loading of the hearing aid user and may result in listening fatigue and ultimately that the user removes the hearing aid(s).
Thus, there is a need for a new hearing aid with improved externalization and localization of sound sources.
A human with normal hearing will also experience benefits of improved externalization and localization of sound sources when using a hearing instrument, such as a headphone, headset, etc, e.g. playing computer games with moving virtual sound sources or otherwise enjoying replayed sound with externalized sound sources.
Human beings detect and localize sound sources in three-dimensional space by means of the human binaural sound localization capability.
The input to the hearing consists of two signals, namely the sound pressures at each of the eardrums, in the following termed the binaural sound signals. Thus, if sound pressures at the eardrums that would have been generated by a given spatial sound field are accurately reproduced at the eardrums, the human auditory system would not be able to distinguish the reproduced sound from the actual sound generated by the spatial sound field itself.
It is not fully known how the human auditory system extracts information about distance and direction to a sound source, but it is known that the human auditory system uses a number of cues in this determination. Among the cues are spectral cues, reverberation cues, interaural time differences (ITD), interaural phase differences (IPD) and interaural level differences (ILD).
The transmission of a sound wave from a sound source to the ears of the listener, wherein the sound source is positioned at a given direction and distance in relation to the left and right ears of the listener is described in terms of two transfer functions, one for the left ear and one for the right ear, that include any linear transformation, such as coloration, interaural time differences and interaural spectral differences. These transfer functions change with direction and distance of the sound source in relation to the ears of the listener. It is possible to measure the transfer functions for any direction and distance and simulate the transfer functions, e.g. electronically, e.g. with digital filters.
If a pair of filters are inserted in the signal path between a playback unit, such as a MP3-player, and headphones used by the listener, the pair of filters having transfer functions, one for the left ear and one for the right ear, of the transmission of a sound wave from a sound source positioned at a certain direction and distance in relation to the listener, to the positions of the headphones at the respective ears of the listener, the listener will achieve the perception that the sound generated by the headphones originates from a sound source, in the following denoted a “virtual sound source”, positioned at the distance and in the direction in question, because of the true reproduction of the sound pressures at the eardrums in the ears.
The set of the two transfer functions, the one for the left ear and the one for the right ear, is called a Head-Related Transfer Function (HRTF). Each transfer function of the HRTF is defined as the ratio between a sound pressure p generated by a plane wave at a specific point in or close to the appertaining ear canal (pL) in the left ear canal and pR in the right ear canal) in relation to a reference (p1). The reference traditionally chosen is the sound pressure pl that would have been generated by a plane wave at a position right in the middle of the head, but with the listener absent. In the frequency domain, the HRTF is given by:HL=PL/P1, HR=PR/P1 
Where L designates the left ear and R designates the right ear, and P is the pressure level in the frequency domain.
The time domain representation or description of the HRTF, i.e. the inverse Fourier transforms of the HRTF, is designated the Head Related Impulse Response (HRIR). Thus, the time domain representation of the HRTF is a set of two impulse responses, one for the left ear and one for the right ear, each of which is the inverse Fourier transform of the corresponding transfer function of the set of two transfer functions of the HRTF in the frequency domain.
The HRTF contains all information relating to the sound transmission to the ears of the listener, including the geometries of a human being which are of influence to the sound transmission to the ears of the listener, e.g. due to diffraction around the head, reflections from shoulders, reflections in the ear canal, transmission characteristics through the ear canals, if the HRTF is determined for points inside the respective ear canals, etc. Since the anatomy of humans shows a substantial variability from one individual to the other, the HRTFs vary from individual to individual.
The complex shape of the ear is a major contributor to the individual spatial-spectral cues (ITD, ILD and spectral cues) of a listener.
In the following, one of the transfer functions of the HRTF, i.e. the left ear part of the HRTF or the right ear part of the HRTF, will also be termed the HRTF for convenience.
Likewise, the pair of transfer functions of a pair of filters simulating an HRTF is also denoted a Head-Related Transfer Function even though the pair of filters can only approximate an HRTF.